High accuracy is required for certain manufacturing functions with respect to various parts of commercial airplanes, including drilling and riveting functions. It is desirable that the end tool location be accurate to within ±0.1 to ±0.3 mm, or in some cases, even more precise. However, the machines to carry out those manufacturing functions are quite expensive and, although necessary to accomplish the desired accuracy, can be commercially burdensome for a manufacturer of such large-scale assemblies.
On the other hand, typical bent-arm, multi-axis type robots used typically in the manufacture of vehicles and similar articles do not require such high accuracy (typical accuracy of ±0.6 mm) but are significantly less expensive than the machines described above used in aircraft manufacture and the like. The robots use a plurality of rotary axes for tool positioning. The controller for the robot determines the desired position/angle of the robot's end tool/end effecter and runs an algorithm to determine the angle of each rotary joint, to properly position the tool. The positioning of the tool is produced by a feedback system using the position of the robot motor/motors. The best possible angular accuracy for such robots is determined by the backlash of the gearbox (at best 0.5 arcminute) and shaft windup, which refers to power transfer components such as shafts and belts. When this is multiplied by the large radial distance to the tool point, 200-1600 mm, a significant error, typically 0.03 to 0.25 mm, results from each joint. With such a positioning system, the overall accuracy of such a robot machine is not sufficient for the aircraft applications noted above and other manufacturing operations requiring high accuracy, such as medical device positioning, welding and machining operations, for example.
In the world of high accuracy machine tools, the XYZ position of the tool is accomplished through the use of linear axes, with secondary feedback. Zero, one or two rotary axes are used in the head of the machine tool, but secondary feedback is not typically used for the rotary axes. The distance to the tool point is quite short, so there is only a small effect in the XYZ positioning of the tool point from the rotary axes and secondary feedback on these axes has a negligible effect on improving accuracy.
Accordingly, it is desirable to be able to control the position of the tool/end effecter of a typical multi-rotary axis manufacturing robot with a significantly higher degree of accuracy then is presently possible with existing robots.